A plasma processing apparatus is used in manufacturing electronic devices such as semiconductor devices and the like. A plasma processing apparatus disclosed in Japanese Patent Application Publication No. 2015-022940 is configured to excite a gas by using a microwave. This apparatus includes a microwave generation unit, an antenna, and a 4E tuner disposed in a waveguide between the microwave generation unit and the antenna. The 4E tuner has four movable short-circuiting plates. By operating the movable short-circuiting plates, impedance matching is automatically performed. Positions of the movable short-circuiting plates after the matching are referred to as tuner positions.
In the case of a plasma processing apparatus using a microwave such as the plasma processing apparatus disclosed in Japanese Patent Application Publication No. 2015-022940, a ceiling plate is consumed by use. In that case, an unstable plasma may be generated. The following is description of estimated mechanism of the generation of the unstable plasma. A resonance frequency exists between the tuner and the plasma. When the ceiling plate is consumed by use, the tuner position is moved. The resonance frequency is changed by the movement of the tuner position. When the changed resonance frequency coincides with a power frequency of a plasma, an unstable plasma is generated. When the unstable plasma is generated, a maintenance operation such as exchange of the ceiling plate or the like is required. Therefore, in this technical field, a microwave control method capable of improving availability of the plasma processing apparatus is required.